1. Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery, a method of preparing the negative active material, and a rechargeable lithium battery including the negative active material. More particularly, the present invention relates to a negative active material having excellent electrochemical reactivity and high energy density per volume, a method of preparing the same, and a rechargeable lithium battery including the same.
2. Description of the Related Art
Lithium rechargeable batteries have recently drawn attention as a power source of small portable electronic devices. Lithium rechargeable batteries use an organic electrolyte solution and thereby have twice the discharge voltage of a conventional battery using an alkali aqueous solution, and accordingly have high energy density.
For positive active materials of a rechargeable lithium battery, lithium-transition element composite oxides capable of intercalating lithium such as LiCoO2, LiMn2O4, LiNiO2, LiNi1−xCoxO2 (0<x<1), and so on, have been researched.
As for negative active materials of a rechargeable lithium battery, various carbon-based materials such as artificial graphite, natural graphite, and hard carbon, which can all intercalate and deintercalate lithium ions, have been used. Since graphite among the carbon-based materials has as low discharge potential as approximately 0.2 V in comparison to lithium, a rechargeable lithium battery using the graphite material having a relatively low discharge potential in comparison to a rechargeable lithium battery using lithium as a negative electrode which shows a high discharge potential at approximately 3.6 V and an excellent energy density characteristic.
Since the graphite negative active material has relatively fine reversibility as well, it is widely used as a power source for small portable electronic devices to provide a rechargeable lithium battery with a long cycle-life.
Negative electrode materials that can provide high power output, boosting charge, long cycle-life, and safety that are better than graphite, however, are required as power sources for middle or large-scale electronic devices such as power tools, power supplies for hybrid electric vehicles (HEVs) or electric vehicles (EVs), uninterruptible power supplies (UPSs), stationary power systems and so on.
To solve this problem, recent technology suggests using oxides to fabricate a negative electrode. For example, U.S. Pat. Nos. 5,545,468 and 6,645,673 disclose using lithium titanium oxides as negative active materials.
Contemporary methods for preparing lithium titanium oxides utilize a wet method, such as a sol-gel method and a hydrothermal synthesis method, or a solid-phase method, to acquire spinel-structured lithium titanium oxide.
Since the solid-phase method may reduce manufacturing costs, compared to the wet method, the solid-phase method is mostly used. The solid-phase method, however, leaves some titanium dioxide unreacted when a heat treatment is performed at a low temperature. In case of a heat treatment performed at a high temperature, the solid-phase method produces byproducts and the particle diameter of generated lithium titanium oxide greatly increases, which leads to a deterioration in physical properties.
In addition, titanium oxide produced through a contemporary preparation method is a porous agglomeration including nano particles. Since the density is very low, energy density per volume decreases.